1. Field of the Invention
The invention relates generally to hyperglycemia and more specifically to methods of detecting pre-diabetes or diabetes in a subject using O-GlcNAc or an antibody that binds thereto.
2. Background Information
Diabetes is characterized by impaired glucose metabolism manifesting itself among other things by an elevated blood glucose level in the diabetic patient. Underlying defects lead to a classification of diabetes into two major groups. Type 1 diabetes, or insulin dependent diabetes mellitus (IDDM), arises when patients lack insulin-producing beta-cells in their pancreatic glands. Type 2 diabetes, or non-insulin dependent diabetes mellitus (NIDDM), occurs in patients with impaired beta-cell function and alterations in insulin action.
The current treatment for type 1 diabetic patients is injection of insulin, while the majority of type 2 diabetic patients are treated with agents that stimulate beta-cell function or with agents that enhance the tissue sensitivity of the patients towards insulin. The drugs presently used to treat type 2 diabetes include alpha-glucosidase inhibitors, insulin sensitizers, insulin secretagogues, and metformin.
Over time, almost one-half of type 2 diabetic subjects lose their response to these agents. Insulin treatment is instituted after diet, exercise, and oral medications have failed to adequately control blood glucose. The drawbacks of insulin treatment are the need for drug injection, the potential for hypoglycemia, and weight gain.
It has become clear that the development of IDDM is known to occur spontaneously in humans, rats and mice over an indeterminate period of time (months to years). Given that the “pre-diabetic” phase of IDDM is long in duration and clinically asymptomatic, one important opportunity for therapeutic intervention falls during this period. However, effective diagnostic assays that can identify people in this pre-diabetic phase are lacking.
There are major limitations in our current criteria for the diagnosis of diabetes. A Fasting Plasma Glucose (FPG)>125 mg/dl reflects only one aspect of glucose metabolism, which may be stated as the post-absorptive balance of hepatic glucose production and peripheral glucose uptake. It does not reflect the free-living, daily glycemic patterns, the prolonged fasted state or the even post-prandial state. The oral glucose tolerance test (OGTT), in addition to being clinically cumbersome, is also non-physiologic (assuming most meal ingestion does not include 75 gm of concentrated sucrose). Assessing glucose tolerance with the single measure of plasma glucose 2 hours after the oral glucose is therefore crude at best. And as noted, the original basis for defining diabetes was the association of FPG and 2-hour Plasma Glucose (PG), to epidemiologic data on microvascular complications of diabetes, mainly in Pima Indians.
The use of hemoglobin Alc (HbAlc) to diagnose diabetes (in addition to its regular use in following the efficacy of treatment) has long been a controversial issue. While important dissent has been raised, a major problem with assays depending upon non-enzymatic glycation and Amadori chemistry is that they are inherently insensitive and not so useful for detection of pre-diabetics and early-onset diabetes. Thus, the official recommendations have stressed the fact that while quite specific for diabetes, if elevated, HbAlc would be a relatively insensitive way to diagnose early diabetes. Therefore HbAlc has not at this point been accepted as part of the official diagnostic criteria.
The discovery of O-linked β-N-acetylglucosamine (O-GlcNAc) more than 20 years ago disproved the long-held dogma that protein glycosylation is restricted to the luminal compartments of the secretory machinery and to the cell surface and extracellular matrix. Early studies of O-GlcNAc's subcellular localization in rat hepatocytes established that it is highly concentrated at the nuclear envelope, particularly at the nuclear pore complex, but is also abundant and widespread within chromatin. However, several cytosolic and cytoskeletal proteins were also found to be glycosylated with O-GlcNAc (O-GlcNAcylated). Later studies of Drosophila polytene chromosomes established that O-GlcNAcylated proteins are abundant throughout chromosomes. The carboxy-terminal domain of a subpopulation of RNA polymerase II is extensively O-GlcNAcylated, and almost all RNA polymerase II transcription factors are modified by the sugar. So far, more than 500 proteins have been identified to be O-GlcNAcylated, and these proteins are involved in almost all aspects of cellular metabolism.
O-GlcNAcylation not only has an important role in many fundamental cellular processes, but also its dysregulation contributes to the aetiology of important human diseases, particularly diabetes. Accordingly, there exists a need for methods that would enable identification of individuals predisposed to diabetes, and would allow treatment early in the disease process, which may help to avert life-long insulin dependence.